Various portable electronic devices such as cell phones and laptops use digital cameras to capture photographs and video and are expected to use digital projectors to display photographs or video on a screen or on an object. These are miniature digital cameras and include a miniature lens assembly, i.e., a lens assembly having a lens diameter of about 0.25 inches or less, which captures the light and focuses it onto a CMOS imager to capture an image. Inside a miniature projector, a lens assembly focuses the light from an LED array onto a screen or an object. The continuing demand for smaller and higher quality low cost imaging lens assemblies presents a considerable challenge to optical and mechanical design. The tiny lenses in the assembly should be aligned with respect to each other or a lens barrel within a few microns to ensure good image quality. Alignment errors between the lenses lead to a reduction in image quality. When the image quality of a lens assembly is not acceptable, the lens assembly is rejected. This leads to undesirable yield loss in the manufacturing of lens assemblies.
To reduce lens alignment errors and improve manufacturing yields of lens assemblies, a variety of passive alignment methods have been devised.
For example, referring to FIGS. 1A and 1B, reproduced from FIGS. 2 and 4, respectively, of U.S. Pat. No. 7,088,530 entitled “Passively Aligned Optical Elements” to Recco et al. In FIG. 1A, the alignment of two lenses L1 and L2 uses mating tapered surfaces 24 and 34. In FIG. 1B, lenses L1 and L3 are aligned to each other using the lens barrel 22. Lenses in this lens assembly are tightly stacked inside the lens barrel 22 into predefined positions and are not allowed to move.
As the resolution of a miniature camera increases, and the performance requirements for the lens assemblies become more stringent, the number of lenses in the lens assembly often increases. The increase in the number of optical elements that are stacked up tends to increase the impact of any alignment errors. As a result, the yield loss in the manufacture of lens assemblies using prior art passive alignment becomes worse when the number of lenses in the assembly increases.
As lens assemblies become smaller, the amount of light collected by the lens assembly is reduced and lower f-number designs are required. The larger aperture designs magnify the sensitivity to lens alignment errors and the yield loss in the manufacture of lens assemblies using prior art passive alignment becomes worse.
Active alignment of lenses is typically used for high performance optical systems where the cost of the active alignment is not an issue. However, known active alignment techniques, such as using an autocollimator and a rotational stage to individually align lenses can be too complex and costly for high-volume production of miniature lens assemblies.
There is a need in the art for a low cost method of manufacturing lens assemblies for use in miniature cameras and miniature projectors that combines the performance advantages of active alignment and the low cost advantages of passive alignment.